The present invention relates to molding. More particularly this invention concerns a mold closing arrangement for an extrusion-type blow molder.
In a common blow molding operation an extruder presses out a tube or parison which is then formed in a mold cavity defined between a pair of mold halves. Gas under pressure is used to force the walls of the parison against the inside of the mold cavity in order to give it the desired hollow shape. Thereafter the element is cooled and, if desired, filled.
Normally each mold half is mounted to one side of a vertical mold plane along which one or more parisons are extruded. It is necessary to force the mold halves together with considerable pressure during the molding operation. Furthermore the mold cavities must be exactly aligned with the parisons in order to prevent some of the extruded material from catching between the faces of the mold not at the cavities and therefore ruining the entire batch.
When the bodies are to be filled and closed in the same machines it has been common practice to use two sets of mold halves which can act on the continuously extruded parisons in a hand-over-hand manner. Thus each pair of mold halves is arranged to be closable scissor-fashion on the parison, and furthermore each of the two pairs of mold halves can be displaced vertically. Thus the one mold half closes on the parisons and it moves down with the continuously extruded material while the other mold half engages above it and similarly moves down. Then the lower mold pair opens and swings up past the still closed upper pair and it closes on the parisons above it. To this end the support for each of the scissor-type molds are on opposite sides of the apparatus in order to allow them to pass each other as they alternate.
In such devices it is usually necessary to pivot each of the mold halves about a vertical axis offset from the mold-closing plane so that the mold axes of each pair symmetrically flank the respective mold closing plane. This type of construction causes the registering recesses that constitute the mold cavity when the two halves lie together to move so that in the last stages of closing they effectively move toward their respective pivot axes in a direction parallel to the mold-closing plane. This movement often results in trapping of some of the material at locations offset from the mold cavity between the molds so that the mold halves cannot close properly. To date the only method of overcoming this difficulty has been to provide a relatively large mold-cavity opening so that the misalignment during closing is compensated for. Such a use of large openings, however, requires considerable redesign of the molding apparatus and greatly increases its cost.